Optical fiber amplifier with oscillating pump energy

ABSTRACT

An optical fiber amplifier has an amplification stage that uses an optical fiber pumped with pump energy of a first wavelength that oscillates through the gain medium. In one embodiment, the pumping is essentially a cavity resonator that is coupled to either end of the optical fiber such that oscillating pump energy is directed into one end of the fiber and out the other, as it reflects back and forth between the ends of the cavity. Highly reflective gratings are used to maintain the oscillation of the pump energy, and a pump energy source, such as pumped doped optical fiber, is coupled to at least one of the gratings. In another embodiment, the pump source comprises multiple reflectors that are employed at each end of the fiber, and independent pump sources each having a slightly different wavelength within the absorption spectrum of the amplifier are coupled together, such that two pump wavelengths are simultaneously oscillated through the gain medium. In another embodiment of the invention, the reflective gratings are integrated directly into a portion of the pathway through which the signal to be amplified passes. This embodiment uses a pump source that causes amplification of the pump energy in the gain medium which, in turn, provides amplification of the optical signal. A two-pass embodiment is also shown in which the optical signal enters and exits from the same optical side of the amplifier fiber. An optical circulator may be used to provide the necessary unidirectional porting of the input and output signals.

REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of patent application Ser. No.08/970,493 filed Nov. 14, 1997.

FIELD OF THE INVENTION

[0002] This invention relates to the field of optical fiber amplifiersand, more particularly, to means of applying pump energy to an opticalfiber amplifier.

BACKGROUND OF THE INVENTION

[0003] As is known in the art, an optical amplifier is a device thatincreases the amplitude of an input optical signal fed thereto. If theoptical signal at the input to such an amplifier is monochromatic, theoutput will also be monochromatic, with the same frequency. Aconventional fiber amplifier comprises a gain medium, such as a singlemode glass fiber having a core doped with a rare earth material,connected to a WDM coupler which provides low insertion loss at both theinput signal and pump wavelengths. The input signal is provided, via thecoupler, to the medium. Excitation occurs through optical pumping fromthe pumping source. Pump energy that is within the absorption band ofthe rare earth dopant is combined with the optical input signal withinthe coupler, and applied to the medium. The pump energy is absorbed bythe gain medium, and the input signal is amplified by stimulatedemission from the gain medium.

[0004] Such amplifiers are typically used in a variety of applicationsincluding, but not limited to, amplification of weak optical pulses suchas those that have traveled through a long length of optical fiber incommunication systems. Optical amplification can take place in a varietyof materials including those materials, such as silica, from whichoptical fibers are formed. Thus, a signal propagating on a silica-basedoptical fiber can be introduced to a silica-based optical fiberamplifier, and amplified by coupling pump energy into the amplifier gainmedium.

[0005] Fiber amplifiers are generally constructed by adding impuritiesto (i.e. “doping”) an optical fiber. For a silica-based fiber, suchdopants include the elements erbium and ytterbium. For example, one typeof fiber amplifier referred to as an erbium (Er) amplifier typicallyincludes a silica fiber having a single-mode core doped with erbium ions(conventionally denoted as Er³⁺). It is well known that an erbiumoptical fiber amplifier operating in its standard so-called three levelmode is capable, when pumped at a wavelength of 980 nanometers (nm), ofamplifying optical signals having a wavelength of approximately 1550nanometers (nm). Likewise, an amplifier having a silica-based fiber“co-doped” with erbium and ytterbium shows excellent amplification of a1550 nm optical signal when pumped with a wavelength of approximately1060 nm. Since 1550 nm is the lowest loss wavelength of conventionalsingle-mode glass fibers, these amplifiers are well-suited for inclusionin fiber systems that propagate optical signals in the wavelengthvicinity of 1550 nm.

[0006] It has been an ongoing pursuit in the field of optical fiberamplifiers to increase the power output of the amplifiers.Traditionally, pump energy is applied to the gain medium by couplinginto the doped fiber either in the same propagation direction as thesignal to be amplified (referred to as “co-pumping”), or by coupling itinto the doped fiber in the opposite direction as the signal to beamplified (referred to as “counter-pumping). Each of these pumpingmethods has its own advantages, but also its own limitations. It is anobject of this invention to go beyond these traditional pumping methodsto provide a high power optical amplifier by providing a new means ofpumping a doped optical fiber.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, an optical fiberamplifier is provided in which a doped optical fiber gain medium ispumped by pump energy that is oscillated through a substantial portionof the gain medium. That is, a resonant cavity for the pump energy isformed that includes the amplifier fiber, such that the pump energy isreflected back and forth through the gain medium. The output couplingfor the resonant cavity is absorption by the doped fiber, which resultsin amplification of the optical signal by stimulated emission as itpasses through the fiber.

[0008] The optical pumping apparatus used to generate the oscillatingpump energy may take a number of different forms. In general, reflectorsthat reflect optical energy at the pump wavelength are coupled to eitherside of the optical fiber, and reflect the pump energy back and forththrough the gain medium. In one embodiment, each reflector is located inits own optical pathway separate from the optical fiber, a first ofthese pathways being coupled to a first side of the optical fiber whilethe second is coupled to the second side of the fiber. The coupling ispreferably by wavelength selective couplers, such as WDMs, so that onlythe pump energy is diverted from the signal path and directed to thereflectors. In one variation of this embodiment, each reflector iscoupled to a pump energy generator, preferably in the form of a pumpedoptical fiber, so that pump energy is generated on either side of theoptical fiber. In another variation, the pump energy is generated atonly one side, while the other side has only a reflector. In eithercase, the pump energy is oscillated in the pathway between thereflectors, providing the desired oscillation of pump energy through thefiber gain medium.

[0009] When the pump energy is coupled into the optical fiber usingwavelength selective couplers, another variation of the inventioninvolves using a plurality of pump wavelengths, each of which is withinthe absorption band of the doped optical fiber. In such an embodiment, aplurality of reflectors may be used in each of the two pump energypathways located, respectively, to either side of the optical fiber. Thedifferent pump wavelengths are preferably close in wavelength, and eachset of reflectors (i.e. each group of reflectors located to one opticalside of the doped fiber) may be coupled together using narrowbandwavelength selective couplers, such as narrowband WDMs. Furthermore,some or all of the reflectors may be coupled to pump sources thatgenerate optical energy at the desired pump wavelengths.

[0010] In each of the above embodiments, the optical fiber may be dopedwith erbium/ytterbium (Er/Yb), which provides amplification of a 1550 nmoptical signal when the fiber is pumped at a wavelength of 1064 nm. Thehighly reflective gratings of the fiber may then be selective to reflectthe 1064 nm wavelength, and the pump sources may themselves be opticalfibers doped, preferably with ytterbium (Yb), and pumped with opticalenergy at a wavelength of, for example, 915 nm.

[0011] In another embodiment of the invention, the reflectors forproviding oscillation of the pump energy through the gain medium areintegrated into a portion of the signal pathway, and may be integratedinto the amplifier fiber itself. These reflectors, preferably highlyreflective Bragg gratings, are wavelength specific, and do notsignificantly interfere with the optical signal to be amplified. Thatis, the reflectors maintain oscillation of optical energy at the pumpingwavelength through the gain medium, while the optical signal passesthrough them and through the doped optical fiber. To cause generation ofenergy at the pump wavelength, a pump source is coupled into the gainmedium and causes amplification of optical energy at the pump wavelengthwithin the gain medium. Thus, the output of the pump source is absorbedby the doped optical fiber, and amplifies the pump energy thatoscillates between the two reflectors. The oscillating pump energy, inturn, is absorbed by the gain medium and amplifies the optical signalpassing through the fiber. In such an embodiment, a ytterbium-dopedfiber may be used. The signal wavelength could then be 1090 nm, the pumpenergy wavelength 1064 nm, and the pump source wavelength 915 nm.

[0012] In one variation of the embodiment having reflectors integratedinto the signal pathway, the amplifier is a two-pass amplifier. A signalreflector is provided at one end of the doped fiber that reflectsoptical energy at the wavelength of the optical signal. The opticalsignal is then coupled through an input port into the other end of thefiber. The optical signal is amplified as it passes through the fiber,which is pumped by oscillating pump energy. Upon reaching the end of thefiber, the optical signal encounters the signal reflector, and isdirected back through the optical fiber, where it is further amplified.At the end of the fiber where it initially entered, the amplifiedoptical signal is coupled out through an output port.

[0013] The foregoing embodiment may be accomplished by using an opticalcirculator, which allows unidirectional coupling of an optical signalfrom one port of the circulator to another. If the optical signal isinput to a first port of the circulator, the amplifier may be located ina branch coupled to a second port, which receives the optical signalfrom the first port. The amplified optical signal, after passing twicethrough the gain medium, returns to the second port of the circulator,where it is coupled to a third port. In one version of this embodiment,the third port is simply a system output port. However, a secondamplifier, identical to that connected to the second port, may belocated in a branch coupled to the third port, and a fourth port of thecirculator could then serve as the system output port. Additionalamplifier branches can also be added in a similar manner up to themaximum port capacity of the circulator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view of a fiber amplifier according to afirst embodiment of the present invention that couples pump sources intoeither side of a gain medium to oscillate pump energy across the gainmedium.

[0015]FIG. 2 is a schematic view of a fiber amplifier according to asecond embodiment of the present invention that couples a pump sourceinto a first side of a gain medium, and reflects pump energy into anopposite side of the gain medium using a periodic grating that is highlyreflective at the pump wavelength.

[0016]FIG. 3 is a schematic view of an alternative embodiment of theinvention which is similar to the embodiment of FIG. 2, but which uses aplurality of coupled pump sources directed into each side of theamplifier gain medium.

[0017]FIG. 4 is a schematic view of another alternative embodiment ofthe invention in which gratings that reflect the pump signal areintegrated directly into the path of the signal to be amplified.

[0018]FIG. 5 is an embodiment similar to that of FIG. 4 that uses anoptical circulator to direct a signal to be amplified into and out ofone or more arms of the circulator, each of which contains anoscillating pump signal in a gain medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Depicted in FIG. 1 is a fiber amplifier having an optical pumpingarrangement that uses two pump sources 30, 32 together to provideoscillation of the pump signal. In the preferred embodiment, an opticalsignal entering the amplifier via input port 10 has a wavelength λ_(S)in the wavelength range of 1550 nm, and is therefore in the peaktransmission range of conventional silica-based optical fiber. From port10, the input signal is directed to wavelength division multiplexer(WDM) 24, by which it is coupled into doped optical fiber 26. The signalis amplified in the fiber 26, and is coupled out of the fiber gainmedium by WDM 28 and directed to output port 34, where the amplifiedsignal may be used in any desired application.

[0020] The optical fiber 26 is doped with a rare earth element, and isthe heart of the fiber amplifier. In the preferred embodiment, the fiberis doped with erbium/ytterbium (Er/Yb) to create the desired gainmedium. Given an Er/Yb doping, the optical fiber 26 may then be pumpedwith optical pump energy in the wavelength range of 1064 nm by thecombination of pump sources 30 and 32, which are coupled into the fiber26 via 1064/1550 WDMs 24 and 28, respectively. Pumping at thiswavelength results in absorption of the pump energy by the doped fiber,and a corresponding amplification of the optical signal within theamplifier gain medium by stimulated emission at the signal wavelength.As discussed below, the arrangement of pump sources 30, 32 as shownprovides pump energy that oscillates back and forth through theamplifier gain medium.

[0021] In the preferred embodiment, each of the pump sources 30, 32 isof identical construction. Pump source 30 consists of double-cladoptical fiber 36, laser diode 40 and highly reflective Bragg grating 42.Pump source 32 consists of double-clad optical fiber 38, laser diode 44and highly reflective Bragg grating 46. Each of the Bragg gratings 42,46 is highly reflective to the pumping wavelength of 1064 nm. The fibers36, 38 are each doped with ytterbium and each has optical energy inputto it by its respective laser diode source 40, 44 at a wavelength in ahigh absorption wavelength range for a Yb-doped fiber. This results in apopulation inversion in each of the pump fibers 36, 38 which, in theresonant cavity arrangement of the two pump sources, results in thedevelopment of pump signal energy at the desired 1064 nm wavelength thatoscillates between the gratings 42, 46. For example, generating pumpenergy at a wavelength of 915 nm with diodes 40, 44 and injecting itinto the double-clad fibers is sufficient to cause the stimulatedemission of optical radiation at 1064 nm. The gratings 42, 46 thenreflect the 1064 nm pump energy back and forth between them, and throughthe optical fiber 26.

[0022] By establishing the pump energy reflection path through theoptical fiber 26 itself, the pump energy at 1064 nm, necessary tocontinuously amplify the 1550 nm optical signal, can be replenishedwithout the risk of destabilization that would exist if the outputs oftwo individual lasers were directed toward each other. For example, thedirection of 1064 nm fiber lasers toward each other has been shown todestabilize both lasers, and cause both to lase at 1106 nm. In thisembodiment, the two gratings 42, 46 combine to form the two ends betweenwhich the pump energy is reflected. The output coupler of the cavity isthe absorption of pump energy by the fiber amplifier. Thus, by havingthe two sources 30, 32 act in concert, the risk of destabilization isremoved.

[0023] In order to maximize the efficiency of the amplifier, therelationship between the absorption by the amplifier and the pumping bythe sources 30, 32 may be exploited. The optimum output transmission fora laser cavity is given by:

T _(opt) =−L _(i)+%(g _(o) L _(i))

[0024] where T_(opt) is the optimum output transmission, L_(i) is theinternal cavity losses, and g_(o) is the unsaturated gain of the lasercavity. Since the fiber amplifier can be considered to be the outputcoupler for the resonant cavity formed by the pump sources 30, 32, andsince the transmission through the fiber amplifier is T=e^(−αR) (where Ris the length of the fiber amplifier), the optimum length for opticalfiber 26 is:

R _(opt)=(−1/α)log[−L _(i)+%(g _(o) L _(i))].

[0025] Shown in FIG. 2 is an alternative embodiment of the presentinvention. Unlike the embodiment of FIG. 1, which uses two pumpedoptical sources 30, 32 as the ends of the pump energy pathway, in FIG.2, one of these sources is replaced with a periodic grating that ishighly reflective at the pump energy wavelength. For example, grating 42may be used to replace pump source 30. As in FIG. 1, the input signalenters port 10, is coupled into the gain medium of doped optical fiber26 via WDM 24, and is coupled out of the gain medium via WDM 28 anddirected toward output port 34. However, while the embodiment of FIG. 1uses two individual pump sources 30, 32, the FIG. 2 embodiment uses onlyone. The high reflectivity of grating 42 at the desired wavelength ofthe pump energy (e.g., 1064 nm) allows it to function as one end of anoscillation path, while pump source 32 acts as the other end. The pumpenergy developed in the fiber 38 at 1064 nm is thus reflected back andforth between grating 46 and grating 42, passing through amplifier fiber26 in the process and providing the desired pumping to the amplifier.

[0026] Shown in FIG. 3 is another alternative embodiment of the presentinvention in which a plurality of fiber laser sources is used for eachof the pump sources 30, 32. In the preferred version of this embodiment,each of the fiber lasers is similar to those used in the FIG. 1embodiment, consisting of a double-clad fiber with a highly reflectiveBragg grating and a laser diode pump. However, the fiber lasers for agiven pump source 30, 32 each have different wavelengths, close to eachother, in the range of 1064 nm. In pump source 30, double-clad fiber 68is Yb-doped, and has a laser diode 70 as a source which provides pumpenergy in the range of, e.g., 915 nm. The grating 72 is selected to behighly reflective at a first wavelength in the 1064 nm range, such as1060 nm. The double-clad fiber 74 and diode 76 can be identical to thefiber 68 and diode 70, respectively, except that Bragg grating 78 ishighly reflective at a wavelength close to, but different from, thewavelength of grating 72. For example, grating 78 may be selected to behighly reflective at a wavelength of 1070 nm. The two differentwavelengths (e.g. 1060 nm and 1070 nm) of pump source 30 are both in thewavelength absorption range of the Er/Yb doped amplifier fiber 26, andare therefore both adequate pump wavelengths for pumping the amplifier.These wavelengths are coupled together into the fiber 26 after beingcombined using narrowband WDM coupler 80, a 1060/1070 WDM.

[0027] In the preferred version of this embodiment, the construction ofpump source 32 is identical to that of pump source 30. Yb-doped,double-clad optical fiber 82 is pumped by diode 84 at a wavelength of,e.g., 915 nm, and is stabilized by highly reflective grating 86 to anoutput wavelength of 1060 nm. Yb-doped, double-clad optical fiber 88 ispumped by diode 90 at a wavelength of, e.g., 915 nm, and is stabilizedby highly reflective grating 92 to an output wavelength of 1070 nm. The1060 nm and 1070 nm wavelengths of the two fiber lasers are combined bynarrowband WDM 94, which is coupled to the opposite end of the amplifierfiber 26.

[0028] In the embodiment of FIG. 3, two overlapping oscillation pathsare established, one for pump energy at 1060 nm and one for pump energyat 1070 nm. WDMs 80 and 94 allow these pump energies to be coupled forpropagation through the optical fiber 26 of the amplifier, andsegregated at the different fibers of each pump source 30, 32. It willbe understood by those skilled in the art that, while the embodiment ofFIG. 5 shows two fiber lasers per pump source, more than two fiberlasers per pump source could also be used. This would require thecoupling of the additional pump energy wavelengths into the fiberamplifier using additional WDMs, but would function according to thesame principles as the construction shown in FIG. 3.

[0029] Shown in FIG. 4 is another alternative embodiment of theinvention, in which the oscillating pump energy used to pump a fiberamplifier is achieved by integrating two pump gratings directly into afiber pathway through which the input optical signal passes. Thispathway may or may not be part of the doped region of the amplifierfiber, but the embodiment removes the need for coupling the pump signalsinto the doped fiber, as is done using WDMs in the foregoingembodiments. As shown, one of the pump reflection gratings 42, 46 ispositioned to either side of doped amplifier fiber 26. The gratings arehighly reflective at the desired pumping wavelength, e.g. 1064 nm.Certain amplifier fibers (e.g., a double-clad, Yb-doped fiber), canserve as the gain medium for generating both the pump energy at 1064 nm,as well as for a desired signal wavelength λ_(S) such as 1090 nm.

[0030] In the embodiment of FIG. 4, initial pumping of the fiber 26 isprovided by optical source 41, which may be a laser diode with an outputwavelength of 915 nm. This pumping energy is coupled into the gainmedium via 915/1090 WDM 27. Absorption of the energy at 915 nm resultsin the development of oscillating pump energy between gratings 42 and 46at the 1064 nm wavelength. That is, an oscillation path for the 1064 nmpump energy is maintained between the two gratings 42, 46. This furtherpumps the fiber gain medium, and allows the input optical signal, at the1090 nm wavelength, to be amplified by stimulated emission as it passesthrough the amplifier fiber. The amplifier optical signal is thereafterdirected to output port 34 via WDM 27.

[0031]FIG. 5 depicts a variation of the embodiment shown in FIG. 4. Asin FIG. 4, gratings 42, 46 are integrated into a signal pathway toeither side of a fiber amplifier 26, doped with, e.g., ytterbium. Thegratings 42, 46 are highly reflective at a desired pumping wavelength,such as 1064 nm, and define the desired oscillation path for the pumpenergy. In the FIG. 5 embodiment, the amplifier and gratings 42, 46 arearranged as a first branch of an optical circulator 35. An opticalcirculator is a commercially available optical coupler that allowsunidirectional one-to-one optical coupling between a set of opticalports. That is, optical energy input to one of the circulator ports isdirected to only one other port, and may only be coupled between thosetwo ports in one propagation direction.

[0032] Also in the signal path with the fiber amplifier is signalgrating 47, which is highly reflective at the wavelength of the desiredoptical signal (e.g., 1090 nm) and is positioned to the side of theamplifier fiber 26 and gratings 42, 46 away from circulator 35. A pumpenergy source 41 is coupled into the optical fiber amplifier. Thissource may be a laser diode having an output wavelength of, e.g., 915nm. The output wavelength of the pump source 41 is absorbed by the dopedfiber 26, and results in the development of an oscillating pump signalbetween gratings 42 and 46 at the 1064 nm wavelength. The 1064 nmwavelength signal, in turn, further pumps the amplifier fiber, allowingit to provide amplification to a 1090 nm signal passing through it bystimulated emission.

[0033] In FIG. 5, the signal to be amplified is directed from input port10 to a first port 43 of the circulator 35. This results in the signalbeing output at a second port 45 of the circulator, where it is directedinto the pumped amplifier arrangement 51. The optical signal isamplified as it passes through the fiber 26, and is thereafter directedto grating 47. Grating 47, being highly reflected at the signalwavelength, redirects the amplified signal back through amplifier fiber26, where it is further amplified. When the amplified signal returns tocirculator port 45, it is directed to circulator port 49.

[0034]FIG. 5 shows a second amplifier stage 51 in the branch connectedto circulator port 49. The second amplifier stage 51 may be identical tothat located in the branch connected to port 45, and provides additionalamplification of the optical signal in the same manner. After the signalpasses through the amplification stage 51 of the second branch, itreturns to circulator port 49, from which it is directed to port 53, andthereafter to signal output port 34. While two amplifier stages 51 areshown in FIG. 5, those skilled in the art will recognize that the numberof stages used is optional. For example, port 49 of the circulator couldas easily lead to signal output port 34, if only one stage ofamplification was desired. Likewise, a circulator with more than fourports could be used, and additional amplification stages 51 beyond thetwo shown in FIG. 5 could be used.

[0035] In the embodiments of the present invention, pump energy isoscillated through the gain medium comprising a fiber amplifier. Thepump energy is thereby directed into both ends of the fiber amplifierwithout the destabilization risk associated with directing twoindependent pump energy sources toward each other. Oscillation of thepump energy through the gain medium also provides recycling of the pumpenergy (as compared with simply passing the pump energy once or twicethrough the gain medium), and therefore helps to improve the overallpower conversion of the amplifier.

[0036] While the invention has been shown and described with referenceto a preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. For example, the pumping arrangements ofthe invention may be applied to amplifiers having different dopingconfigurations and desired pump wavelengths.

What is claimed is:
 1. An optical amplifier system for amplifying anoptical signal at a signal wavelength, comprising: at least one firstpump source to provide first pump light at a first wavelength; a firstoptical fiber having a first active material for generating second pumplight at a second wavelength and including a lasing resonator betweenspatially disposed reflectors forming a second pump laser source, thesecond pump source optically coupled to receive the first pump lightfrom the first pump source, the first optical fiber absorptive of thefirst pump light and providing gain to the second pump light; and asecond optical fiber having a second active material forming a fiberamplifier at the signal wavelength, the second optical fiber opticallycoupled within the lasing resonator of the second pump laser source tolaunch the second pump light into the second optical fiber, the secondoptical fiber absorptive of the second pump light and providing gain tothe signal wavelength.
 2. The optical amplifier system of claim 1further comprising a first pump source at each end of the second pumplaser source.
 3. The optical amplifier system of claim 1 wherein thesecond optical fiber comprises a double clad fiber having a core dopedwith a rare earth material through which the signal wavelength to beamplified propagates, and an inner cladding surrounding the core forreceiving the second pump light via the second pump laser source.
 4. Theoptical fiber amplifier of claim 1 wherein the reflectors comprise fiberBragg gratings reflective of the second pump light.
 5. The optical fiberamplifier of claim 1 further comprising a second lasing resonator insaid second optical fiber so that the fiber amplifier functions as afiber laser.